CHAPTER X.[ToC]
CONCLUSION.
In this concluding chapter, I propose to give a summary of the results at which we have arrived from the study of recent earthquakes, and this can, I think, be done best by describing what may be regarded as an average or typical earthquake, though it may be convenient occasionally to depart slightly from such a course. Few shocks have contributed more to our knowledge than the majority of those described in this volume; but, on certain points, we gain additional information from the investigation of other earthquakes, and these are referred to when necessary for the purpose in view.
FORE-SHOCKS.
At the outset, we are met by a question of some interest and great practical importance—namely, whether there are any constant signs of the coming of great earthquakes by means of which their occurrence might be predicted and their disastrous effects mitigated.
Excluding the Ischian earthquakes, which belong to a special class, it is evident that there is generally some slight preparation for a great earthquake. For a few hours or days beforehand, weak shocks and tremors are felt or rumbling noises heard within the future meizoseismal area. But, unfortunately, it has not yet been found possible to distinguish these disturbances from others of apparently the same character which occur alone, so that for the present they fail to serve as warnings.
In Japan, where the organisation of earthquake-studies is more complete than elsewhere, it is possible that a vague forecast might be made, if the distribution of the fore-shocks of the earthquake of 1891 should prove to be a general feature of all great earthquakes. It was at first supposed that this earthquake occurred without preparation of any kind; but a closer analysis of the records shows that during the previous two years there was a very decided increase in the seismic activity of the district, and also that the distribution of the epicentres marked out the future fault-scarp, and at the same time exhibited a tendency to comparative uniformity over the whole fault-region.
For the present, then, the only warning available is that given by the preliminary sound, which may precede the strongest vibrations by as much as five or ten or even more seconds. Though two or three seconds may elapse before its character is recognised, the fore-sound thus allows time for many persons to escape from their falling houses. Some races, however, are less capable of hearing the sound than others, and this may be one reason why Japanese earthquakes are so destructive of human life.